Enhancement mode gallium nitride power devices

ABSTRACT

Enhancement mode III-nitride devices are described. The 2DEG is depleted in the gate region so that the device is unable to conduct current when no bias is applied at the gate. Both gallium face and nitride face devices formed as enhancement mode devices.

BACKGROUND

This disclosure is related to gallium nitride based semiconductortransistors.

Gallium nitride (GaN) semiconductor devices, which are III-V orIII-nitride type devices, are emerging as an attractive candidate forpower semiconductor devices because the GaN devices are capable ofcarrying large currents and supporting high voltages. Such devices arealso able to provide very low on-resistance and fast switching times. Ahigh electron mobility transistor (HEMT) is one type power semiconductordevice that can be fabricated based on GaN materials. As used herein,GaN materials that are suitable for transistors can include secondary,tertiary, or quaternary materials, which are based on varying theamounts of the III type material of AlInGaN, Al, In and Ga, from 0 to 1,or Al_(x)In_(y)Ga_(1-x-y)N. Further, GaN materials can include variouspolarities of GaN, such as Ga-polar, N-polar, semi-polar or non-polar.In particular, N-face material may be obtained from N-polar orsemi-polar GaN.

A GaN HEMT device can include a III-nitride semiconductor body with atleast two III-nitride layers formed thereon. Different materials formedon the body or on a buffer layer causes the layers to have differentband gaps. The different materials in the adjacent III-nitride layersalso causes polarization, which contributes to a conductive twodimensional electron gas (2DEG) region near the junction of the twolayers, specifically in the layer with the narrower band gap. One of thelayers through which current is conducted is the channel layer. Herein,the narrower band gap layer in which the current carrying channel, orthe 2DEG is located is referred to as the channel layer. The device alsoincludes a gate electrode, a schottky contact and an ohmic source anddrain electrodes on either side of the gate. The region between the gateand drain and the gate and source, which allows for current to beconducted through the device, is the access region.

The III-nitride layers that cause polarization typically include abarrier layer of AlGaN adjacent to a layer of GaN to induce the 2DEG,which allows charge to flow through the device. This barrier layer maybe doped or undoped. In some cases, doping of the barrier layer may addto channel charge and it may also help in dispersion control. Because ofthe 2DEG typically existing under the gate at zero gate bias, mostIII-nitride devices are normally on or depletion mode devices. If the2DEG is depleted, i.e., removed, below the gate at zero applied gatebias, the device can be an enhancement mode or normally off device.

Enhancement mode or normally off III-nitride type devices are desirablefor power devices, because of the added safety they provide. Anenhancement mode device requires a positive bias applied at the gate inorder to conduct current. Although methods of forming III-nitrideenhancement type devices are known, improved methods of depleting the2DEG from under the gate in the channel layer are desirable.

SUMMARY

Devices are described that are enhancement mode devices with low offstate leakage current as well as low on resistance. This is achieved instructures that result in not only depleting the 2DEG from under thegate region, but also have a high barrier to current flow under the gateregion in the off state while ensuring that the region outside the gate,i.e., the access region, remains highly conductive.

In one aspect, a method of forming an N-face enhancement mode highelectron mobility transistor device is described. The method includesforming on a substrate a Ga-faced sacrificial layer, forming a cap layeron the sacrificial layer, forming a GaN channel layer on the cap layer,forming an Al_(x)GaN layer on the channel layer, wherein 0≦x≦1, forminga buffer layer on the Al_(x)GaN layer, bonding a carrier wafer on thebuffer layer to form a stack, removing the substrate and the sacrificiallayer from the stack to form an N-faced assembly of layers and forming agate, source and drain on the N-faced assembly of layers.

In another aspect, a normally off III-nitride HEMT device is described.The device includes a gate, a source and a drain and an access regionformed of a III-nitride material between either the source and the gateor the drain and the gate. In the access region the sheet resistance isless than 750 ohms/square. The device has an internal barrier under thegate of at least 0.5 eV, such as at least 1 eV, when no voltage isapplied to the gate. The device is capable of supporting a 2DEG chargedensity under the gate of greater than 1×10¹²/cm² in the on state.

In yet another aspect, a Ga-face enhancement mode high electrodemobility transistor device is described. The device includes a GaNbuffer layer, a p-type bottom cap on the GaN buffer layer, wherein theGaN buffer layer has an aperture exposing the bottom cap, a GaN channellayer on an opposite side of the bottom cap from the GaN buffer layer,an Al_(x)GaN layer on an opposite side of the GaN channel layer from thecap layer, a p-type top cap on an opposite side of the Al_(x)GaN layerfrom the channel layer and a gate adjacent to the top cap.

In yet another aspect, a method of making a Ga-face enhancement modehigh electrode mobility transistor device is described. The methodincludes forming a structure including the GaN buffer, GaN channel layerand Al_(x)GaN layer, forming the p-type top cap on the Al_(x)GaN layer,forming the gate adjacent to the p-type top cap, applying a passivationlayer over the p-type top cap and Al_(x)GaN layer, bonding a carrierwafer onto the passivation layer and forming the aperture in the GaNbuffer layer.

In another aspect, a Ga-face enhancement mode high electrode mobilitytransistor device is described. The device has a GaN buffer layer, anAl_(x)GaN layer on the GaN buffer layer, wherein the GaN buffer layerhas an aperture exposing the Al_(x)GaN layer, a GaN channel layer on anopposite side of the Al_(x)GaN layer from the GaN buffer layer, anAl_(y)GaN layer on an opposite side of the GaN channel layer from theAl_(x)GaN layer, wherein a gate region of the Al_(y)GaN layer is treatedwith fluorine and an upper gate adjacent to the gate region. Thefluorine treatment can include a treatment with a fluorine containingplasma.

In yet another aspect, a method of forming a Ga-face enhancement modehigh electrode mobility transistor device is described. The methodincludes forming a structure of the GaN buffer layer, the Al_(x)GaNlayer on the GaN buffer layer, wherein the GaN buffer layer has anaperture exposing a portion of the Al_(x)GaN layer, a GaN channel layeron an opposite side of the Al_(x)GaN layer from the GaN buffer layer andan Al_(y)GaN layer on an opposite side of the GaN channel layer from theAl_(x)GaN layer, treating the exposed portion of the AlxGaN layer with afluorine containing compound and treating the gate region of the AlyGaNlayer with the fluorine containing compound.

In yet another aspect, a structure that is part of an enhancement modehigh electrode mobility transistor device is described. The structureincludes a GaN buffer layer on a substrate. On the buffer layer is aheterostructure region and 2DEG formed by a layer of AlGaN, with analuminum composition between 0 and 1 or equal to 1 and a GaN channellayer. A cap is on the layers that form the heterostructure region. Adielectric layer is formed on the layers that form the heterostructureregion and adjacent to the cap. A gate on the cap. The device is anN-face device.

In one aspect, an N-face enhancement mode high electron mobilitytransistor device is described. The device includes a substrate and aheterostructure region and 2DEG region formed by a layer of AlGaN withan aluminum composition between 0 and 1 or equal to 1 and a GaN channellayer. The heterostructure region is on the substrate. The GaN channellayer has a Ga-face adjacent to the layer of Al_(x)GaN. A cap is in arecess of an N-face of the channel layer. The cap does not overlie anaccess region of the device. A gate is formed on the cap. A source anddrain are on laterally opposing sides of the cap.

Embodiments of the devices and methods described herein may include oneor more of the following features. A GaN channel layer on the cap layercan be a channel layer of GaN with up to 15% Al in the GaN. The caplayer can include p-type Al_(z)GaN and a method of forming a device canfurther include etching the p-type Al_(z)GaN to form a p-type Al_(z)GaNcap, where forming a gate includes forming the gate on the p-typeAl_(z)GaN cap. Forming the channel layer and forming the Al_(x)GaN layeron the channel layer can form a region of a first 2DEG charge, a methodcan further include forming a layer surrounding the p-type Al_(z)GaNcap, the layer surrounding the p-type Al_(z)GaN cap and the channellayer together having a net 2DEG charge that is greater than the first2DEG charge. Forming a layer surrounding the p-type Al_(z)GaN cap caninclude forming a layer of Al_(y)GaN, wherein y<x. Forming a cap layerof p-type Al_(z)GaN can include forming the cap layer to have athickness of at least 50 Angstroms, with 0<z<1. Forming a channel layerof GaN can comprise forming a channel layer having a thickness less than300 Angstroms under the gate region. Forming a GaN channel layer caninclude forming a channel layer having a thickness about 50 Angstroms. Adevice can have a 2DEG charge that is depleted under the gate and canhave an internal barrier that is greater than 0.5 eV, such as at least 1eV. The channel layer can be Al_(z)GaN, 0.05<z<0.15. Forming the caplayer can include forming a multi-compositional cap layer, wherein afirst layer of the cap layer comprises Al_(x)GaN and a second layer ofthe cap layer comprises of Al_(y)GaN, wherein the second layer is formedprior to the first layer being formed and y>x. A method of forming adevice can include etching the multi-compositional cap layer to form amulti-compositional cap and forming a layer of GaN surrounding themulti-compositional cap. The surrounding GaN layer can be formed usingselective regrowth. The multi-compositional cap layer can change fromAl_(x)GaN to Al_(y)GaN in a continuous or discontinuous manner.

The carrier layer can be thermally conducting and electricallyinsulating. Removing the substrate can include using laser liftoff,lapping, wet etching or dry etching. The method can further includeplasma treating a portion of an N-face that corresponds to a location inwhich the gate is subsequently formed. The channel layer and the layerof Al_(x)GaN can form a heterostructure with a resulting 2DEG region inthe channel layer and the method can further include implanting ions inthe access region of the wider bandgap layer to increase net 2DEGcharge. The device can have an access region, and the method can furtherinclude doping the access region by thermal diffusion of donor species.An N-face layer can be passivated after the N-face layer is exposed. Ina device where the structure is built upside down, an AlN layer can beformed on the channel layer prior to forming the layer of Al_(x)GaN. Theaccess region can be selectively doped in the channel layer, such as bythermal diffusion of donor species. A dielectric layer can be formed ona surface of the access region to form a pinning layer. The device canbe capable of blocking at least 600 V, 900 V or 1200 V. The device canhave an on-resistance of less than 15 mohm-cm², less than 10 mohm-cm², 3mohm-cm² or less than 2 mohm-cm². A top cap can be formed of p-typeAl_(z)GaN. The top cap may comprise a thin AlN layer, e.g., less than 20Angstroms, or a high Al composition AlGaN layer, e.g., where the Alcomposition is greater than 50%, to prevent or reduce gate leakage. Abottom cap can be formed of p-type GaN. The bottom cap can be formed ofAl_(y)GaN, wherein y varies from one surface to an opposite surface ofthe bottom cap. The Al_(x)GaN layer can have a thickness of less than500 Angstroms. The channel layer can have a thickness of less than 300Angstroms, such as less than 100 Angstroms, under the gate region. Agate can be in an aperture and contacting the bottom cap. A layer ofAl_(y)GaN can laterally surround the top cap, where y>x. The device canhave an internal barrier of at least 0.5 eV, such as at least 1 eV whenno voltage is applied to the gate. A gate can be formed in the aperturein the GaN buffer layer. The Al_(x)GaN layer exposed by an aperture canbe doped with fluorine. A lower gate can be within an aperture exposingthe Al_(x)GaN layer. A p-type cap layer can be between the upper gateand the Al_(y)GaN layer. A p-type cap layer can be between the lowergate and the Al_(x)GaN layer. An insulator layer can be between thelower gate and the Al_(x)GaN layer. An insulator layer can be betweenthe upper gate and the Al_(y)GaN layer. The device can have an internalbarrier of at least 0.5 eV, such as at least 1 eV, when no voltage isapplied to the gate. An insulator can be between the upper gate and thegate region. A cap can be a p-type cap. The cap can be a combination ofp-type AlGaN layer and an AlN layer. The cap can include Al_(y)GaN andAl_(x)GaN, the Al_(y)GaN is closer to the gate than the Al_(x)GaN is andy>x. The Al_(y)GaN and Al_(x)GaN can be doped p-type. The channel layercan be adjacent to the cap. The dielectric layer can be a dopantdiffusion layer and donor species in the dopant diffusion layer canincrease 2DEG density in the access region. The dielectric layer can beon a side of the cap opposite to the channel layer. The channel layercan be adjacent to the cap and can have an N-face adjacent to the cap.The dielectric layer can form a pinning layer and can induce charge inthe access region. A layer of AlN can be between the layer of AlGaNforming the heterostructure and the 2DEG and the GaN channel layer. Aslant field plate can be on the gate. The dielectric layer can bebetween the cap layer and the gate. The GaN channel layer can laterallysurround a gate region in which the gate is located. The cap can be ap-type Al_(z)GaN. The cap can include p-type Al_(z)GaN and AlN layers.The cap can include Al_(y)GaN and Al_(x)GaN, where the Al_(y)GaN iscloser to the gate than the Al_(x)GaN is and y>x. The Al_(y)GaN andAl_(x)GaN can be doped p-type. The cap can include Al_(y)GaN andAl_(x)GaN, where the Al_(y)GaN is closer to the gate than the Al_(x)GaNis, Al_(y)GaN and Al_(x)GaN are doped p-type and x>y. An access regionbetween the gate and source and between the gate and drain can be ionimplanted. An insulating layer can be disposed between the gate and thecap. A dielectric passivation layer can be over at least the accessregion.

Implementations of the methods and devices described herein can includeone or more of the following advantages. High performance normally offdevices with high positive threshold voltage are achieved. The positivethreshold voltage can be adjusted by depositing an insulator of varyingthickness on a device. However, high performance normally off devicesrequire a large internal barrier that is not easily adjusted by merelydepositing a thick insulator. A device can be formed with a highbarrier, which determines the off state leakage current when the deviceis off. The internal barrier under the gate can be greater than 1 eV.The device may have a threshold voltage that is between about 1-3 volts.A device with a high internal barrier under the gate region can beformed while ensuring adequate charge or 2DEG in the access regions. Thecharacteristics of the gate and access region can be independentlycontrolled. Thus, a high internal barrier, high threshold voltage andlow access region-resistance (high access region conductance) cansimultaneously be achieved.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of a double p-type capped device.

FIG. 2 is a band diagram under the gate region of the double p-typecapped device.

FIG. 3 is a band diagram in the access region of the double p-typecapped device.

FIGS. 4-13 show intermediary structures created while forming the doublep-type capped device.

FIG. 14 is a schematic representation a device with a regrown AlGaNlayer surrounding a p-type cap.

FIG. 15 is a schematic representation a device with a dopant inducinglayer in the access region.

FIG. 16 is a schematic representation of a device treated with afluorine based plasma on both the Ga-face and the N-face.

FIG. 17 is a band diagram under the gate region of the fluorine treateddevice.

FIG. 18 is a band diagram in the access region of the fluorine treateddevice.

FIG. 19 is a GaN crystal structure with a Ga-face.

FIG. 20 is a schematic representation of a III-nitride typeheterostructure of a Ga-face device.

FIG. 21 is a GaN crystal structure with an N-face.

FIG. 22 is a schematic representation of a III-nitride typeheterostructure of an N-face device.

FIG. 23 is a schematic representation of an N-face device with a p-typecap under the gate.

FIG. 24 is a band diagram under the gate region of the N-face devicewith a p-type cap under the gate.

FIG. 25 is a schematic representation of a device with a layer ofvarying thickness in the access region.

FIG. 26 is a band diagram in the access region of the N-face device witha p-type cap under the gate.

FIGS. 27-29 show an exemplary method of forming an N-face enhancementmode device with a p-type cap.

FIG. 30 is a schematic representation of a device with amulti-compositional cap.

FIG. 31 is a band diagram under the gate region of the device with amulti-compositional cap.

FIG. 32 is a band diagram in the access region of the device with amulti-compositional cap.

FIGS. 33-37 show an exemplary method of forming the enhancement modeN-face device with the multi-compositional cap.

FIG. 38 is a schematic representation of a device that has been ionimplanted in the access region.

FIGS. 39-41 illustrate p-type cap devices and structures.

FIG. 42 is a schematic representation of a device with a layer forselectively doping the access region.

FIGS. 43-44 are schematic representations of a device with a Fermipinning layer.

FIG. 45 is a schematic representation of a device with a dielectric cap.

FIG. 46 is a schematic representation of a device with a dielectricpassivation layer on the N-face.

FIG. 47 is a schematic representation of a device with a slant fieldplate.

FIG. 48 is a schematic representation of a device with a layer of AlNbetween the layers of the heterojunction.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Ga-Faced Devices

Referring to FIG. 1, a Ga-face enhancement mode III-nitride device isformed as a lateral device having a gate 17, source 18 and drain 19, thelateral device formed on a Ga-face surface of the III-nitride device andgating on both the Ga and N-faces of the device. In some embodiments,the Ga-face is a Ga-polar face. The device includes a p-type Al_(z)GaNcap 11 on the Ga-face and a p-type Al_(y)GaN cap layer (shown as p GaNlayer 14) accessed from the N-face of the device, where 0<y<1 and 0<z<1.In some embodiments, the p-type Al_(z)GaN cap 11 has a thickness ofbetween about 1 nm and 100 nm, such as about 10 nm. In some embodiments,the p-type Al_(y)GaN cap layer has a thickness of between about 1 nm and30 nm, such as about 10 nm. In some embodiments, either of the p-typeAl_(z)GaN cap 11 or the p-type Al_(y)GaN cap layer is a continuouslygraded layer, that is, includes more or less aluminum at differentdepths of the layer. The device includes a GaN layer 15 on the N-face ofthe p GaN layer 14. The GaN layer 15 includes a recess which exposes thep GaN layer 14 and can have a thickness of between about 10 nm and 500nm, such as about 50 nm.

On an opposite side of the p GaN layer 14 is a GaN channel layer 13,which can have a thickness of between about 1 nm and 50 nm, such asabout 10 nm. A layer of Al_(x)GaN 12 adjacent to the GaN channel layer13 and opposite to the p-type GaN cap layer 14 contributes to the 2DEGin the GaN channel layer 13. The p-type Al_(z)GaN cap 11 is on the layerof Al_(x)GaN 12, in the gate region and under gate 17. The break in the2DEG indicator line under the gate, shows that there is no charge underthe gate at zero bias on the gate and that the device is an enhancementmode or normally off device. (A dashed line in each figure, other thanFIGS. 20 and 22, indicates the 2DEG) Each of the III-nitride layers canbe epitaxially grown on one another.

In some embodiments, the Al_(x)GaN/GaN layers 12, 13 are grown thinenough so that the surface pinning position of the p-type Al_(z)GaN orGaN layers 11, 14 depletes the 2DEG at the Al_(x)GaN/GaN layer interfacein the gate region. For example, the Al_(x)GaN/GaN layers 12, 13 aregrown thin when the device includes a fully depleted p-type layer. Ifthe device has a thick p-type layer on top, the barrier created by thep-type Al_(z)GaN/Al_(x)GaN junction depletes the 2DEG Depleting the 2DEGfrom both surfaces increases the internal barrier and the thresholdvoltage. The presence of high p-Al_(x)GaN or GaN barriers also resultsin high gate turn-on voltage and reduction of gate leakage current.Additional insulating layers may be applied between the gates (17 or17′) and the respective p-type layers (11 and 14).

In some embodiments, one of gates 17, 17′ is optional on the device.

Without the p-type Al_(z)GaN cap 11, the polarization fields in theAl_(x)GaN/GaN layers 12, 13 allows for the 2DEG at the Al_(x)GaN/GaNinterface in the access region. Thus, the thickness of the Al_(x)GaN cap12 is controlled to maintain adequate 2DEG and a low on-resistance.

Referring to FIGS. 2-3, the band diagrams of the double p-type capdevice under the gate and the access region, respectively, show theconduction band (E_(C)) and valence band (E_(V)) with respect to theFermi level (E_(F)). In the band diagram, the minimum distance 90between the conduction band E_(C) and the Fermi level E_(F) at zero biason the gate defines the device's internal barrier. The internal barrierof the device is about 1 eV. Referring to FIG. 3, the conduction bandE_(c) crosses the Fermi level E_(F) in the access region of the device,indicating that the device has a high 2DEG density in the access regionand hence can achieve a low on-resistance.

Referring to FIGS. 4-13, the method of forming a device with a p-typecap on both sides is described. Referring to FIG. 4, a GaN buffer layer15, a p-type GaN cap layer 14 (or p-type Al_(y)GaN (0<y<1)), a GaNchannel layer 13, a Al_(x)GaN layer 12 and a p-type Al_(z)GaN cap layer9 are epitaxially grown on a substrate 16. The exposed surface of thep-type Al_(z)GaN cap layer 9 and the top surface of each layer isGa-faced. Referring to FIG. 5, the p-type Al_(z)GaN cap layer 11 isetched to define a gate region where the p-type Al_(z)GaN cap 11 islocated. Referring to FIG. 6, a gate 17 with a schottky contact andsource 18 and drain 19 with ohmic contacts are formed on the Ga-face toform an assembly. Referring to FIG. 7, a passivation layer 23 is appliedto the exposed Ga-faces of the assembly. Referring to FIG. 8, a bondinglayer 24 is applied to the passivation layer 23. Referring to FIG. 9, acarrier wafer 60 is attached to the bonding layer 24. Referring to FIG.10, the substrate 16 is removed from the GaN layer 15. Within the GaNbuffer layer an additional sacrificial layer can be included (notshown). This layer contains an etch stop layer which is not etched whenthe sacrificial layer is etched. When the sacrificial layer is used,after etching the substrate the sacrificial layer is etched selectivelyto ensure a planar N-face GaN surface. Referring to FIG. 11, theassembly is then flipped over so that the N-face is accessible.Referring to FIG. 12, the GaN buffer layer 15 is etched to form a recessthat allows for access to the p-type GaN cap layer 14. A gate 17′ isthen deposited on the N-face of cap layer 14 within the recess andcontacting the p-type GaN cap layer 14. Referring to FIG. 13, back sidecontact is made to the original front side source 18 and drain 19 ohmiccontacts. Additionally, contact is made with the gate pad (not shown,because it is in the plane of the figure) and any other pads wherecontact is required.

Referring to FIG. 14, in an alternative embodiment of the device of FIG.1, a layer of Al_(y)GaN 20, where y>x, is grown in the access regionsurrounding the p-type cap. The layer of Al_(y)GaN 20 can furtherenhance the 2DEG and conductivity under the access region. Thus theon-resistance may be lower than in the device shown in FIG. 1. Themethod of forming this device is similar to that described in FIG. 4-13with the exception of selectively growing the layer of Al_(y)GaN 20after the etch of the p-type layer 11.

Referring to FIG. 15, in yet another variation of the device of FIG. 1,the access regions are doped. An intermediary step of making the deviceis shown. The doping is achieved by thermal diffusion of donor speciesinto the access regions. A thin film dielectric 29 with a donor species,such as Si, SiO₂ or SiN_(x) (in the case of Si dopant), is depositedonto the Ga-face access region of a III-nitride epitaxial layerstructure. The thin film dielectric 29 can either be applied to theGa-face or the N-face of the device. The material is annealed, such asat a temperature between about 300 and 900° C., which increases the 2DEGdensity in the access regions, thus resulting in lower on-resistance. Insome embodiments, multiple diffusions are performed to mimic a lightlydoped drain structure. In some embodiments, the thin film dielectric isremoved after the annealing process.

Referring to FIG. 16, another embodiment of an enhancement modeIII-nitride device with gating on both the Ga and N-faces is shown wherethe device is fluorine treated. In some embodiments, the Ga-face is aGa-polar face and the N-face is an N-polar face. The device includes aregion on both the Ga-face beneath the gate 17 and a region on an N-facebeneath the gate region that has been treated with a fluorine compound.The fluorine treatment can be a fluorine based plasma treatment. Afluorine treatment on both the Ga-face and the N-face increases theinternal barrier and the threshold voltage of the device.

The structure under the gate is a layer of Al_(y)GaN 25 on a GaN channellayer 13 on a layer of Al_(x)GaN 21, which is on a GaN buffer layer 15.A recess in the GaN buffer layer 15 exposes a portion of the layer ofAl_(x)GaN 21. The recess is below the gate 17 and not below the accessregion. The exposed portion of the layer of Al_(x)GaN 21 is treated witha plasma of a fluorine compound. Similarly, a gate region of the layerof Al_(y)GaN 25 is treated with the plasma. The fluorine-based treatmentis not applied to the access regions.

In some embodiments, a bottom gate 17′ is formed in the recess after thefluorine treatment of the N-face. In some embodiments, the Alcomposition, x, in the Al_(x)GaN layer 21 is minimized, such as tobetween 0.1 to 0.3, for example, 0.1, and the thicknesses of the GaNchannel layer 13 and the Al_(x)GaN layer 21 are controlled to preventdepletion of the 2DEG or the formation of a parasitic channel at theinterface between the layer of Al_(x)GaN 21 and GaN buffer layer 15. Insome embodiments, the GaN channel layer 13 has a thickness of about 20nm. In some embodiments, the thickness of the Al_(x)GaN layer 21 is 10nm. Optionally, the device includes an insulator 27 between the gate 17and the layer of Al_(y)GaN 25 and/or between the bottom gate 17′ and thelayer of Al_(x)GaN 21. The insulator can have a thickness of betweenabout 0.1 nm and 100 nm. In some embodiments, one of the gates 17, 17′is optional.

Referring to FIG. 17, the band diagram under the gate regions of adevice with fluorine treatment under N-face and Ga-face gates shows aninternal barrier of 0.8 eV (at minimum distance 90). A possiblemechanism of the shift in threshold voltage on which the band diagram isbased is that F ions act as acceptors. The fluorine-based plasmatreatment results in high gate turn-on voltage and reduction of gateleakage current. Referring to FIG. 18, the band diagram in the accessregion shows a high 2DEG that will result in a low on-resistance.

In some embodiments, the fluorine based plasma treatment is combinedwith the device shown in FIG. 1, a p-type cap device. The fluorine basedtreatment can be applied to both the Ga-face and N-face surface.Alternatively, one surface can be p capped and the opposite surface canbe treated with a fluorine treatment. This combination results in adevice with a high internal barrier and a high threshold voltage whilemaintaining low on-resistance.

N-Face Devices

Referring to FIG. 19, a number of the devices described above are formedas Ga-face devices. A Ga-face device has a crystal structure withgallium atoms on its exposed face. A Ga-face structure can be Ga-polar,semi-polar or a non-polar GaN structure. Referring to FIG. 20, when alayer of AlGaN is deposited onto a layer of GaN, a 2DEG automaticallyforms because of the built-in sheet charge and electric fields in theheterostructure. Thus, Ga-face devices naturally tend to form depletionmode devices. The methods described above allow the Ga-faced devices tobe enhancement mode devices. Many conventional III-nitride type devicesare Ga-faced because a Ga-faced device can be easier to grow.

Referring to FIG. 21, a device with a crystal structure with N atomsexposed or on its face is referred to as an N-face device. The devicecan be N-polar, semi-polar or non-polar. Referring to FIG. 22, when alayer of AlGaN is deposited onto a layer of GaN, there is no spontaneouspolarization in the heterostructure that causes the device to be adepletion mode device. Therefore, N-faced devices can be more easilymade enhancement mode.

Referring to FIG. 23, an N-face device is formed with a p-type cap underthe gate. The device has an epitaxial layer structure of a substrate 16that includes a GaN layer 15, an Al_(x)GaN layer 43 and a GaN channellayer 41 (bottom to top). The 2DEG is in the GaN channel layer 41. Ap-type cap 11 of Al_(z)GaN, 0<z<1, is grown thick enough, such as atleast 10 Angstroms, or in some instances as thin as p-type materialgrowth allows, so that the raised barrier height due to the surfacepinning position of the p-Al_(z)GaN and barrier induced by thep-Al_(z)GaN/GaN hetero-interface depletes the 2DEG at the GaN channellayer 41/Al_(x)GaN layer 43 interface under the gate region at zero gatebias. Because the p-Al_(z)GaN increases the gate barrier height, thegate turn-on voltage increases and the gate leakage current decreases.To further reduce gate leakage, a thin, e.g., a layer less than 100Angstroms, AlN layer can be included under the p-type Al_(z)GaN cap andabove the GaN channel 41 in the gate region. This AlN layer can also bewithin the p-type cap. In some embodiments the AlN layer is doped p-typeor is an Al_(w)GaN layer with a high Al composition (w>x).

In some embodiments, the GaN channel layer 41 is reduced to 5 nm toincrease the internal barrier and the threshold voltage of the device.Referring to FIG. 24, the internal barrier of the device is at least 1.5eV (at minimum distance 90).

Referring to FIG. 25, in some embodiments, the GaN channel layer 41 isthicker in the access region than in the gate region. Because the 2DEGdensity increases with the thickness of the GaN channel layer, theon-resistance of the device can be reduced by increasing the thicknessof GaN channel layer 41. Thus, the GaN channel layer 41 can be grown tosurround the p-type Al_(z)GaN cap 11. The GaN channel layer 41 can alsoextend under the Al_(z)GaN cap 11, but it can be thicker in the accessregion, up to 500 nm, such as about 30 nm. The thin portion under thegate can be about 5 nm in thickness.

Reducing the thickness of the GaN layer under the gate increases thebarrier under the gate and hence, the threshold voltage. The thickportion in the access region allows for sufficient 2DEG at the GaNchannel layer 41/Al_(x)GaN layer 43 interface to result in minimumresistance in the access region. In some embodiments, the full thicknessof the GaN channel layer 41 is grown first and then subsequently etchedaway, followed by the selective regrowth of the p-type Al_(z)GaN cap 11.In other embodiments, a thinner GaN channel layer 41 is formed duringthe first structure growth and is then capped by a layer of Al_(z)GaN,followed by etching the layer of Al_(z)GaN outside the gate region,i.e., in the access region and the regrowth of the remainder of GaNchannel layer 41 in the access region. Referring to FIG. 26, the energyband diagram in the access region is shown. Without the high barrier ofthe p-Al_(z)GaN cap 11, polarization in these layers allow for a 2DEG atthe GaN channel/Al_(x)GaN interface outside the gate region.

In alternative embodiments to the device shown in FIGS. 23 and 25, thep-type Al_(z)GaN cap 11 is doped, such as with Mg or other p-typedopant. In some embodiments, the p-type Al_(z)GaN cap 11 is a gradedlayer where z changes gradually, such as from 0 to 0.5. In someembodiments, in the p-type Al_(z)GaN cap 11, z is 0.3 and has athickness of about 5 nm. In some embodiments, the GaN channel layer 41includes a small fraction of aluminum, thus forming a layer ofAl_(y)GaN, where y is less than 0.15. The small amount of Al can improvethe breakdown voltage of the device.

As noted above, formation of an N-face device is not necessarily as easyas growing a Ga-face device. Referring to FIGS. 27-29, a method offorming an N-face device, such as the device shown on FIG. 23, isdescribed, wherein the original layers are grown as Ga-face layers andthen flipped to realize the intended N-face device. Referring to FIG.27, an epitaxial layer structure is grown in substrate 50. The epitaxiallayer structure includes a thick GaN layer 55, an Al_(x)GaN layer 43, aGaN channel layer 41, a p-type Al_(z)GaN layer 9, and a GaN buffer 52,which are on the substrate 50 (from top to bottom). The epitaxial layerstructure is grown as a Ga-face structure and is subsequently flipped.Thus, the thick GaN layer 55 eventually will serve as the buffer layerof the N-face device.

Referring to FIG. 28, a carrier wafer 60 is bonded onto the thick GaNlayer 55 to form an assembly. The bond can be a metal based bond or adielectric bond or other suitable bond. If the carrier wafer 60 willeventually serve as the final substrate, the carrier wafer can bethermally conducting and electrically insulating. In some embodiments,the bond between the carrier wafer 60 and the thick GaN layer 55 is notconductive.

Referring to FIG. 29, the assembly is flipped over so that the carrierwafer 60 is on the bottom of the device. The substrate 50 is removedusing a technique suitable for the substrate material, such as laserliftoff for sapphire substrates, lapping or plasma etching for SiC basedsubstrates or wet or dry etching for silicon substrates. The GaN bufferlayer 52 is also removed, such as by a dry etch. The structure is now anN-face structure that is ready for completing to form the devices shownin FIGS. 27 and 30.

Referring to FIG. 30, in some embodiments, a multi-compositional cap 65is formed under the gate of an N-face device. The epitaxial layerstructure of the device is a channel layer of GaN 41 on a layer ofAl_(z)GaN 44 on GaN buffer layer 15, which is on substrate 16. The GaNchannel layer 41 is thicker in the access region than in the gateregion. In the gate region, a cap 65 is formed with either a gradedcomposition of AlGaN or multiple layers of AlGaN, such as a layer ofAl_(x)GaN that is adjacent to the channel layer and a layer of Al_(y)GaNthat is adjacent to the gate 17, where y>x. In some embodiments, x=0.3and y=0.5, and each of Al_(x)GaN and Al_(y)GaN are 5 nm thick. If themulti-compositional cap is graded, the grading can change from x to y ina continuous or discontinuous manner. The polarization and bandgapdifferences in the multi-compositional AlGaN layers increase the barrierheight and deplete the 2DEG at the interface between the GaN channellayer 41 and the layer of Al_(z)GaN 44 in the gate region at zero gatebias. As in FIG. 25, reducing the GaN channel layer thickness in thegate region also increases the threshold voltage. Further, because themulti-compositional AlGaN cap increases the gate barrier height, thegate turn-on voltage increases and the gate leakage current decreases inthe device. Referring to FIG. 31, the multi-compositional cap incombination with the thinned GaN channel layer portion in the gateregion can result in a device with at least a 1.4 eV internal barrier.Referring to FIG. 32, the access region of the device shows a high 2DEGconcentration that enables low on-resistance.

Referring to FIG. 33, the device in FIG. 30 can be formed by startingwith an epitaxial layer structure of a substrate 50 on which a GaNbuffer layer 15, a layer of Al_(z)GaN 44, a GaN channel layer 41, alayer of Al_(x)GaN 67 and a layer of Al_(y)GaN 69 are formed (frombottom to top). The structure is an N-face device. The Al_(y)GaN layer69 and Al_(x)GaN layer 67 are then etched to form the cap 65. AdditionalGaN material is regrown around the cap 65, above the GaN channel 41. Thegate 17, source 18 and drain 19 contacts then are formed. Alternatively,a structure with a thick GaN channel layer 41, a layer of Al_(z)GaN 44and a GaN layer 15 are epitaxially grown on substrate 16, as shown inFIG. 34. The thick GaN channel layer 41 is etched in the gate region andthe cap 65, comprising of the materials of the layer of Al_(x)GaN 67 andthe layer of Al_(y)GaN 69, is regrown (see FIG. 30).

Similar to the method shown in FIGS. 27-29, the multi-compositional capdevice can also be formed by forming a Ga-face structure and flippingthe structure. Referring to FIG. 35, an epitaxial layer structureincluding a thick GaN layer 55, a layer of Al_(z)GaN 44, a GaN channellayer 41, a layer of Al_(x)GaN 67, a layer of Al_(y)GaN 69 and a GaNbuffer 52 are formed on the substrate 50 (from top to bottom). Theepitaxial layer structure is grown as a Ga-face and is subsequentlyflipped. Thus, the thick GaN layer 55 eventually will serve as thebuffer layer of the N-face device.

Referring to FIG. 36, a carrier wafer 60 is bonded onto the thick GaNlayer 55 to form an assembly. The bond can be a metal based bond or adielectric bond. If the carrier wafer 60 will eventually serve as thefinal substrate, the carrier wafer can be thermally conducting andelectrically insulating. However, the bond between the carrier wafer 60and the thick GaN layer 55 is not conductive.

Referring to FIG. 37, the assembly is flipped over so that the carrierwafer 60 is on the bottom of the device. The substrate 50 is removedusing a suitable method for the type of substrate, such as laser liftofffor sapphire substrates, lapping or plasma etching for SiC basedsubstrates or wet or dry etching for silicon substrates. The GaN bufferlayer 52 is also removed, such as by a dry etch. The structure is now anN-face structure that is ready for completing to form the device shownin FIG. 30.

In some embodiments, the layers of Al_(x)GaN 67 and Al_(y)GaN 69 areomitted in the initial growth and the GaN channel layer 41 is the finaldesired thickness for the access region, when it is applied to the layerof Al_(x)GaN 67. The GaN channel layer 41 is then etched in the gateregion and the cap 65 is formed where the GaN channel layer material wasremoved. In some embodiments, the device shown in FIG. 30 is formedwithout the regrown GaN material on the GaN channel layer 41. Thus,there is no recess in which the cap 65 is located.

Referring to FIG. 38, in some embodiments, an N-face device is formed byion implanting the access region with n-type dopant ions, such as Si.Although only the device of FIG. 38 is shown as being doped, otherstructures described herein can be doped using this method. The accessregion portion of the Al_(x)GaN layer 21 is ion implanted to increasethe 2DEG density at the interface of the GaN channel layer 13 and theAl_(x)GaN layer 21, outside the gate region. The resulting bandstructure in these regions force the electrons from the dopant ions tofall in the 2DEG at the interface of the GaN channel layer 13 and theAl_(x)GaN layer 21.

Referring to FIGS. 39-41, alternative ways of forming a device with ap-type cap are shown. The p-type cap layer is a layer of Al_(y)GaN 25which extends from the source 18 to the drain 19 contacts. The layer GaN31 is etched in the gate region to form layer GaN 32 and a gate isdeposited such that the layer of GaN 32 is surrounding the gate 17 andon the p-type cap layer of Al_(y)GaN 25. Hence, the layer of GaN 32 isonly in the access region and increases the 2DEG in that region. The caplayer of AlyGaN 25 where not covered by the layer of GaN of the gateregion, depletes the 2DEG to realize normally off operation. Source anddrain contacts 18, 19 are deposited to complete the device, as shown inFIG. 39.

The devices shown in FIGS. 39 and 41 can be formed by starting with astack of a GaN layer 15, a layer of Al_(x)GaN 21, a GaN channel layer 13and a layer of p-type Al_(y)GaN 25, and a GaN layer 32, all grown onN-face layers. The polarization provided by the layer of GaN 32 in thisN-face structure contributes to increasing the 2DEG. In someembodiments, the layer of GaN 32 has a thickness of at least 10 nm andthick enough to ensure charge in the access region. Referring back toFIG. 39, the layer of GaN 32 is etched in the gate region and left toremain in the access region. A gate electrode is deposited on the p-typeAl_(y)GaN layer 25 in the gate region. The etch back of the layer of GaNto form layer of GaN 32 results in depletion of the 2DEG under the gateregion, making the device normally off. Source 18 and drain 19 ohmiccontacts are also deposited.

Referring to FIG. 41, in some embodiments, an insulating layer 35 isdeposited, such as by MOCVD, PECVD, ICP, E-beam or other suitabledeposition method, over the etched layer of GaN 32 and between the gate17 and the layer of p-type Al_(y)GaN 25. The insulating layer 35 canfurther reduce gate leakage current increase gate turn on voltage andprovide passivation. The insulating layer can be deposited either beforeor after forming the ohmic contacts for the source 18 and drain 19. Ifthe insulating layer 35 is deposited before the formation of the ohmiccontacts, portions of the layer can be removed or left in place wheremetallization is to be deposited for the ohmic contacts. Both the deviceof FIG. 39 and FIG. 41 device do not require a regrowth step.

Referring to FIG. 42, an N-face device is formed that has a selectivelydoped access region. The access region is doped by thermal diffusion ofdonor species from a dielectric (or other suitable) dopant diffusionlayer 75 in the access region. The dopant diffusion layer 75 can includedonor species, such as Si, SiO₂, SiN_(x) and other suitable donorspecies. The dopant diffusion layer 75 is annealed to cause the dopant(Si in case of Si, SiO_(x) or SiN_(x)) to migrate into the device andincrease the 2DEG density in the access regions, thereby causing thedevice to have a lower on-resistance. The thermal diffusion can becarried out at any suitable temperature, for example between about 300and 1000° C. To enhance the breakdown voltage of the device, multiplediffusions can be performed to mimic a lightly doped drain structure. Insome embodiments, the dopant diffusion layer 75 is removed afterannealing.

Referring to FIG. 43, in an alternative embodiment to the device in FIG.42, instead of a dopant diffusion layer 75, a dielectric layer whichfunctions as a Fermi level pinning layer 78 is applied on the device.The pinning layer 78 can be either doped or undoped. The pinning layer78 induces charge in the access region. Referring to FIG. 44, in someembodiments the pinning layer is not only in the access region, but isalso formed on the p-type Al_(z)GaN cap 11. The cap 11 blocks anyeffects from the pinning layer 78 on the device in the gate region andthus the pinning can be on the cap without adversely causing a 2DEG inthe gate region. The pinning layer 78 can be a layer of SiN_(x), such asa layer of SiN_(x) grown by MOCVD, PECVD, CATCVD or other suitablemeans, including a combination of various deposition techniques. SiN_(x)on N-face or Ga-face III-nitride devices can pin the surface Fermi levelclose to the conduction band, resulting in high electron concentrationand increased conductivity under the SiN_(x) region. The pinning layer78 can be deposited at any suitable step in the fabrication sequence ofthe device, such as before the ohmic metal contacts are deposited orafter. The pinning layer can be removed from the gate, source or draincontacts where electrical contact will be made.

FIGS. 45-48 show a variety of features that can be used with any of thedevices described herein. Although the devices shown are N-face devices,the features can also be used with Ga-face devices.

Referring to FIG. 45, a SiN_(x) cap 80 can be applied to the N-face ofthe cap during an early stage of processing and is selectively removedat a desired step in the process. N-face III-nitride devices can be moresusceptible to damage than Ga-face III-nitride devices. Thus, theSiN_(x) cap 80 serves to protect the N-face surface from undesireddamage during processing. The SiN_(x) cap 80 can be thin, such as lessthan 2000 Angstroms, for example, 100 Angstroms. In some embodiments,part of the SiN_(x) cap 80 is left in the gate region to function as agate insulator.

Referring to FIG. 46, in some embodiments, a dielectric passivationlayer 83 is formed on an N-face device. The passivation layer 83 can beSiN_(x) or other suitable passivation material. The passivation materialcan be deposited by CVD, such as PECVD, MOCVD or ICP or by evaporation.In addition, an optional field plate 87 is formed over the gate regionto reduce the peak electric field and help trapping and breakdownvoltage capacity. The field plate 87 can be terminated at the source orat the gate.

Referring to FIG. 47, a slant field plate 93 can be applied to an N-facedevice. The slant field plate 93 maximizes breakdown voltage. The slantfield plate can be applied along with a dielectric passivation layer 83.In some embodiments, such as the embodiment shown, the slant field plate93 is integrated with the gate.

Referring to FIG. 48, a device can be formed with a gate insulator 96and/or an AlN inter-layer 97 between the GaN channel layer 41 and theAl_(x)GaN layer 43. The gate insulator 96 is between the gate and thetop semiconductor layer, such as the p-type cap or the channel layer.The gate insulator can minimize gate leakage. The gate insulator 96 isformed of a suitable insulating material, such as SiN_(x), SiO₂ or AlN.The layer of AlN 97 between the GaN channel layer 41 and the Al_(x)GaNlayer 43 is thin, such as greater than 0 to about 30 Angstroms. Thislayer improves the mobility-2DEG density product, resulting in a devicewith lower resistance.

Throughout the specification and in the claims, where III-nitridematerials are described, a modification of the material may be used inits place so long as the material is not modified in such a way toreverse the intended polarization, e.g., by hindering the 2DEG in anaccess region or by inducing charge in the gate region. For example,where use of GaN is described, small amounts of aluminum or indium,e.g., up to 15%, 10%, 5% or 2% may be included in the GaN layer withoutdeviating from the scope of the disclosed methods and devices.Similarly, where AlGaN materials are described, AlInGaN materials can beused in their place. That is, any of the GaN materials that aredescribed can be replaced by secondary, tertiary, or quaternarymaterials, which are based on varying the amounts of the III typematerial of AlInGaN, Al, In and Ga, from 0 to 1, orAl_(x)In_(y)Ga_(1-x-y)N. When Al_(x)GaN material is described, 0<x<1,Al_(x)Ga_(1-x)N can be substituted. Further, when a subscript for agroup III material is used in the specification, such as x, y or z, adifferent letter may be used in the claims. Throughout thespecification, ≧ or < may be substituted by ≧or ≦, respectively and ≧ or≦ can be substituted by > or <, respectively.

Throughout the specification and in the claims, the Al_(x)GaN layeradjacent to the channel layer and responsible for forming aheterostructure with and 2DEG in the channel layer, can be doped atleast in part. In embodiments, the doping is n-type. Throughout thespecification, the GaN buffer layer is generally semi-insulating but insome embodiments may include a small portion, such as a portion furthestfrom the substrate side of the buffer layer, that is doped. This dopingcan be either n-type or p-type.

The devices described herein can be formed on a substrate of sapphire,silicon carbide (either Si-face or C-face), silicon, aluminum nitride,gallium nitride or zinc oxide. Although not shown in the variousepilayer structure schematics, a transition layer or a nucleation layercan be formed on the substrate to facilitate the growth of theIII-nitride layers. The nucleation layer is specific to the type ofsubstrate used.

In many embodiments, a cap is only in the gate region and not in theaccess region. However, in other embodiments, the cap extends across theaccess region as well.

Reference is made to fluorine treatment throughout the specification.This treatment may result in fluorine doping in the semiconductorlayers.

Many intermediary structures are described herein, which aresubsequently finished by depositing a gate metal and source and drainohmic contacts. Further, individual devices can be isolated whenmultiple devices are formed on a single substrate. Where these steps arenot explicitly stated, it is assumed that one would finish the deviceusing known techniques.

The transistors described herein are power transistors, which arecapable of blocking at least 600 V, such as at least 900 V or at least1200 V.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, many of the features described with one embodiment may be usedwith another embodiment. Accordingly, other embodiments are within thescope of the following claims.

1. A Ga-face enhancement mode high electrode mobility transistor device,comprising: a GaN buffer layer; a p-type bottom cap on the GaN bufferlayer; a GaN channel layer on an opposite side of the bottom cap fromthe GaN buffer layer; an Al_(x)GaN layer on an opposite side of the GaNchannel layer from the cap layer; a p-type top cap on an opposite sideof the Al_(x)GaN layer from the channel layer; a top gate adjacent tothe top cap; a source and a drain; and an access region between eitherthe source and the gate or the drain and the gate.
 2. The device ofclaim 1, wherein the top cap is formed of p-type Al_(z)GaN.
 3. Thedevice of claim 1, wherein the bottom cap is formed of p-type GaN. 4.The device of claim 1, wherein the bottom cap is formed of Al_(y)GaN,wherein y varies from one surface to an opposite surface of the bottomcap.
 5. The device of claim 1, wherein the Al_(x)GaN layer has athickness of less than 500 Angstroms.
 6. The device of claim 1, whereinthe channel layer has a thickness of less than 300 Angstroms.
 7. Thedevice of claim 1, further comprising a layer of Al_(y)GaN laterallysurrounding the top cap, where y>x.
 8. The device of claim 1, whereinthe device has an internal barrier of at least 0.5 eV when no voltage isapplied to the gate.
 9. The device of claim 1, wherein the GaN bufferlayer has an aperture exposing the bottom cap.
 10. The device of claim9, further comprising a bottom gate in the aperture and contacting thebottom cap.
 11. The device of claim 1, wherein the access region isdoped.
 12. The device of claim 11, wherein the access region is dopedwith a donor species.
 13. A method of making the device of claim 1,comprising: forming a structure including the GaN buffer, GaN channellayer and Al_(x)GaN layer; forming the p-type top cap on the Al_(x)GaNlayer; forming the top gate adjacent to the p-type top cap; applying apassivation layer over the p-type top cap and Al_(x)GaN layer; bonding acarrier wafer onto the passivation layer; and forming the aperture inthe GaN buffer layer.
 14. The method of claim 13, further comprisingforming a bottom gate in the aperture in the GaN buffer layer.